1. Field of the Invention
The present invention relates to recording apparatus for and method of picture image which improves picture quality, particularly the reproducibility of fine lines, as a result of recording the data by varying the gradation depending upon whether the data to be recorded consists of lines or is a photograph.
2. Description of the Prior Art
A known recording method consists of converting picture data into electric signals, and recording picture image by the electric signals, as employed in some electrophotographic reproducing machines and facsimiles. In this kind of recording method, the electric signals which represent picture data are treated in the form of binary (1, 0) signals from the standpoint of easy signal processing, and the picture is recorded as a black-and-white picture. When it is desired to record the picture with gradations in several stages from white to black, however, the electric signals are treated to produce intermediate tones between white and black. Various methods for recording the picture with gradation have heretofore been known, such as (1) a simulation screen process in which the size (diameter) of black dots forming the picture is changed, or the density of dots of a predetermined size is changed, (2) a Dither method in which the level of picture elements in the picture signals is compared with a predetermined threshold value, and the signals are converted into binary values (0, 1) depending upon the level, and (3) a density pattern method in which the kind of dot pattern is switched to express light and shade depending upon the portions where the brightness of the picture changes steeply and the portions where the brightness of the picture changes mildly, in order to clearly present contours.
Among the above-mentioned gradation recording methods, the Dither method further includes a systematic Dither method in which numerical values (threshold values) sampled at equal intervals up to a predetermined value are arrayed according to a predetermined rule to form a matrix (Dither matrix), levels of picture elements in the picture signals are compared with numerical values at corresponding positions in the matrix, and whether or not the picture elements should be recorded is determined relying upon the level. It has been known that the systematic Dither method can easily offer picture of good quality. The emphasize contrast in the gradation method based on the systematic Dither method, threshold values constituting the matrix are increased and decreased uniformly or increased by predetermined values. To obtain fine gradations in which light and shade change smoothly by this method, however, complicated pre-treatment must be carried out.
In order to simply process the picture in real time, therefore, a method of recording a picture image has been contemplated, according to which provision is made of a plurality of matrices which consist of a plurality of threshold values with predetermined frequency distribution arrayed according to a predetermined order, the level of each picture element in the electric signals representing the picture data is compared with each threshold value in the matrix, and the picture is recorded based upon the picture signal which is produced when the level of the picture elements is greater than the threshold value.
The picture image of the original to be recorded may consist of a picture (line) made up of fine lines, or a picture (photograph) having a wide range of density regions, or a combination of these two kinds of pictures. According to the above-mentioned method of recording a picture image, however, a suitable matrix is selected among Dither matrices which have been prepared beforehand depending upon whether the picture of the original pertains to a line picture of a photograph, the level of each picture element in the electric signals representing the picture data is compared with each threshold value in the selected Dither matrix, and the picture is recorded based on binary signals which are formed relying upon the compared results. Therefore, there is no problem when the picture is either a line picture or a photograph. But when the picture consists of a combination of a line picture and a photograph, selection of a Dither matrix suited for either one of them deteriorates the quality of the other part, making it difficult to maintain high quality over the whole picture to be recorded.